1. Field of Invention
The present invention relates to a movable stage apparatus capable of precise movement, and particularly relates to a stage apparatus movable in one linear direction capable of high accuracy positioning and high speed movement, which can be especially favorably utilized in a microlithographic system. This invention also relates to an exposure apparatus that is used for the transfer of a mask pattern onto a photosensitive substrate during a lithographic process to manufacture, for example, a semiconductor element, a liquid crystal display element, a thin film magnetic head, or the like.
2. Description of Related Art
When a semiconductor element or the like is manufactured, a projection exposure apparatus is used that transfers an image of a pattern of a reticle, used as a mask, onto each shooting area on a wafer (or a glass plate or the like) on which a resist is coated, used as a substrate, through a projection optical system. Conventionally, as a projection exposure apparatus, a step-and-repeat type (batch exposure type) projection exposure apparatus (stepper) has been widely used. However, a scanning exposure type projection exposure apparatus (a scanning type exposure apparatus), such as a step-and-scan type, which performs an exposure as a reticle and a wafer are synchronously scanned with respect to a projection optical system, has attracted attention.
In a conventional exposure apparatus, a reticle stage, which supports and carries the reticle, which is the original pattern, and the wafer to which the pattern is to be transferred, and the driving part of the wafer stage are fixed to a structural body that supports a projection optical system. The vicinity of the center of gravity of the projection optical system is also fixed to the structural body. Additionally, in order to position a wafer stage with high accuracy, the position of the wafer stage is measured by a laser interferometer, and a moving mirror for the laser interferometer is fixed to the wafer stage.
Furthermore, in order to carry a wafer to a wafer holder on the wafer stage, a wafer carrier arm that takes out a wafer from a wafer cassette and carries it to the wafer holder, and a wafer carrier arm that carries the wafer from the wafer holder to the wafer cassette, are independently provided. When the wafer is carried in, the wafer that has been carried by the wafer carrier arm is temporarily fixed to and supported by a special support member that can be freely raised and lowered and that is provided on the wafer holder. Thereafter, the carrier arm is withdrawn, the support member is lowered, and the wafer is disposed on the wafer holder. After this, the wafer is vacuum absorbed to the top of the wafer holder. When the wafer is carried out from the exposure device, the opposite operation is performed.
As described above, in the conventional exposure apparatus, the driving part of the wafer stage or the like and the projection optical system are fixed to the same structural body. Thus, the vibration generated by the driving reaction of the stage is transmitted to the structural body, and the vibration is also transmtted to the projection optical system. Furthermore, all the mechanical structures were mechanically resonate to a vibration of a predetermined frequency, so there are disadvantages such that deformation of the structural body and the resonance phenomenon occurred, and position shifting of a transfer pattern image and deterioration of contrast occurred when this type of vibration is transmitted to the structural body.
Furthermore, because the wafer stage moves over a long distance from the carrier arm for carrying in and out of the wafer to the exposure position, it is necessary to provide an extremely long moving mirror for the laser interferometer. Because of this, the weight of the wafer stage becomes relatively heavy and the driving reaction becomes large because a heavy motor with a large driving force is needed. Furthermore, in order to improve throughput, when the moving speed and acceleration of the stage needs to be increased, the driving reaction becomes even larger. In addition, as the weight and acceleration of the stage increase, the heating amount of the motor increases, and there is a disadvantage such that measurement stability or the like of the laser interferometer deteriorates.
Furthermore, in the case of carrying the wafer into and out of the exposure apparatus, the wafer is temporarily fixed and supported on the top of a special support member, so carrying in and out of the wafer consumes time. This causes deterioration of throughput. Additionally, as one example, because giving and receiving of the wafer is performed between the carrier arms, the probability of the wafer being contaminated is high, and the probability of having an operation error when the wafer was given and received is high. Furthermore, the number of carrier arms is a major point governing the size of the carrier unit, so the carrier path becomes long when giving and receiving of the wafer is performed between the carrier arms on the carrier path. Additionally, a floor area (foot print) that is needed for the exposure apparatus also becomes large.
In wafer steppers, the alignment of an exposure field to the reticle being imaged affects the success of the circuit of that field. In a scanning exposure system, the reticle and wafer are moved simultaneously and scanned across one another during the exposure sequence.
To attain high accuracy, the stage should be isolated from mechanical disturbances. This is achieved by employing electromagnetic forces to position and move the stage. It should also have high control bandwidth, which requires that the stage be a light structure with no moving parts. Furthermore, the stage should be free from excessive heat generation which might cause interferometer interference or mechanical changes that compromise alignment accuracy.
Commutatorless electromagnetic alignment apparatus such as the ones disclosed in U.S. Pat. Nos. 4,506,204, 4,506,205 and 4,507,597 are not feasible because they require the manufacture of large magnet and coil assemblies that are not commercially available. The weight of the stage and the heat generated also render these designs inappropriate for high accuracy applications.
An improvement over these commutatorless apparatus was disclosed in U.S. Pat. No. 4,592,858, which employs a conventional XY mechanically guided sub-stage to provide the large displacement motion in a plane, thereby eliminating the need for large magnet and coil assemblies. The electromagnetic means mounted on the sub-stage isolates the stage from mechanical disturbances. Nevertheless, the combined weight of the sub-stage and stage still results in low control bandwidth, and the heat generated by the electromagnetic elements supporting the stage is still substantial.
Even though the current apparatus using commutated electromagnetic means is a significant improvement over prior commutatorless apparatus, the problems of low control bandwidth and interferometer interference persist. In such an apparatus, a sub-stage is moved magnetically in one linear direction and the commutated electromagnetic means mounted on the sub-stage in turn moves the stage in the normal direction. The sub-stage is heavy because it carries the magnet tracks to move the stage. Moreover, heat dissipation on the stage compromises interferometer accuracy.
It is also well known to move a movable member (stage) in one long linear direction (e.g. more than 10 cm) by using two of the linear motors in parallel where coil and magnet are combined. In this case, the stage is guided by some sort of a linear guiding member and driven in one linear direction by a linear motor installed parallel to the guiding member. When driving the stage only to the extent of extremely small stroke, the guideless structure based on the combination of several electromagnetic actuators, as disclosed in the prior art mentioned before, can be adopted. However, in order to move the guideless stage by a long distance in one linear direction, a specially structured electromagnetic actuator as in the prior art becomes necessary, causing the size of the apparatus to become larger, and as a result, generating a problem of consuming more electricity.